1. Field of the Invention
An aspect of this disclosure relates to a cleaning method and a processing apparatus.
2. Description of the Related Art
When performing plasma processing on a wafer, it is important to control radicals in plasma to improve in-plane uniformity of the wafer. A system proposed for this purpose includes a gas diffusion chamber that is formed in an upper electrode and is divided from the center into multiple zones including a center zone and an edge zone. The system can separately control supply of gases into the multiple zones (see, for example, Japanese Laid-Open Patent Publication No. 2009-283715). In this system, gas control can be performed in various manners by, for example, introducing additive gases into desired zones of the multiple zones.
To introduce a gas into multiple zones, a flow splitter (or split-flow controller) for dividing the flow of the gas is necessary. There are two types of flow splitters: a pressure-control flow splitter and a flow-rate control flow splitter. The pressure-control flow splitter makes it possible to control pressures in the respective zones.
Non Plasma Particle Cleaning (NPPC) is a method for removing particles in a chamber. NPPC is a cleaning method in which N2 gas is introduced into a chamber at a high flow rate and the pressure in the chamber is controlled at a predetermined level or higher so that dust is blown off by a shock wave of the gas and ejected from the chamber. When a pressure-control flow splitter is used during NPPC, it is possible to control the pressure in a chamber using a pressure control function of the flow splitter.
However, with a pressure-control flow splitter, when an additive gas is added to a gas after the flow of the gas is divided (i.e., at a downstream position), the pressure in a zone to which the additive gas is added becomes different from the pressure in a zone to which the additive gas is not added. This may influence the accuracy of split flow control. For this reason, when a pressure-control flow splitter is used, it is necessary to limit the flow rate of an additive gas added at a downstream position.
On the other hand, with a flow-rate control flow splitter that directly controls the flow rate of gas, the influence of a pressure change at a downstream position is small and it is possible to supply a greater amount of an additive gas compared with a pressure-control flow splitter.
However, a flow-rate control flow splitter includes a laminar flow tube. A laminar flow tube has a very narrow flow path to measure a flow rate and has a small conductance. For this reason, even when the pressure is controlled at a position upstream of the laminar flow tube to make the pressure in a chamber reach a level that is necessary to perform NPPC, a pressure loss occurs at the laminar flow tube in the flow splitter and the pressure in the chamber located downstream of the laminar flow tube may become lower than the level necessary to perform NPPC. This in turn may reduce the effect of removing particles by NPPC.